Electrochemical valorisation of carbon dioxide to carbon containing renewable fuels and materials has had special attention in the literature over the last years. Recent research efforts have been focussed on some studies with filter-press type cells.
A filter-press electrolytic cell is an electrolytic cell consisting of several units in series. The basic unit cell contains an electrode pair (anode and cathode), with controlled narrow interelectrode gap by using dielectric spacers, rubber gaskets (for liquid and gas tightening) and membrane separators (if needed). The assembly is compressed between metal (usually stainless steel) end plates using tie-rods. In addition, spacers can hold plastic turbulence promoters adjacent to electrodes (to enhance mass transport).
The configuration of the cell allows providing reliable data to aid the scale up to industrial production scale electrolysers, using similar materials, interelectrode gap and space velocities.
In an electrolytic cell, CO2 is reduced on the cathode while the oxygen evolution reaction takes place on the anode. Some examples of half reactions of the cathode for electrochemical CO2 reduction into products are shown below:CO2+2H++2e−→CO+H2OCO2+H++2e−→HCOO−CO2+8H++8e→CH4+2H2O2CO2+12H++12e−→C2H4+4H2O
Formic acid is one of the relevant possible products in the electrolysis of aqueous solutions of CO2, the main product of the reaction depending on the used electrocatalyst. Nevertheless, a problem in the utilization of CO2 in aqueous solution derives from its low solubility in water at standard temperature and pressure. Higher pressures are necessary to increase the CO2 concentration in the liquid phase, but electrode stability in these conditions is limited.
Solvents with high solubility for CO2 are used in the nonaqueous electrochemical reduction of CO2. However, high CO2 solubility requires larger current density, but low electrolytic conductivity leads to high ohmic losses.
Another problem is that high current densities are necessary to increase the productivity and to maximize the formation of hydrocarbons, and also a fast deactivation is present in these conditions.
The use of porous electrodes such as gas diffusion electrodes (GDEs) on electrochemical reduction of CO2 to formate using filter-press type cells have been object of recent studies. The GDEs allows operation at higher current densities and also permits direct feed of gaseous CO2 to the cell.
Alvarez-Guerra M. et al., AlChE Journal, 2014, 60(10) pp. 3557-3564, describe the influence of key variables on the performance of an experimental system under dark conditions for continuous electroreduction of CO2 to formate with a gas diffusion electrode (GDE) loaded with Sn as electrocatalyst.
There are still several technological challenges in CO2 electrocatalytic reduction. The low catalyst activity of the catalysts employed makes the overpotential in CO2 electroreduction too high, and as a consequence, energy efficiency is not good enough for industrial exploitation of the process. Furthermore, the catalysts used suffer from insufficient stability and durability.
This is because the catalysts become gradually covered by reaction intermediates and by-products blocking catalyst active sites. Finally, scaling-up these processes need of strong efforts on optimizing system designs and the electrodes/reactor.